One of the primary steps in the fabrication of modern semiconductor devices is the formation of an insulation film on a semiconductor substrate. Such insulation films are used for a variety of purposes including, among others, separating adjacent conductive layers (e.g., an intermetal dielectric (IMD) layer separates adjacent metal lines while a premetal dielectric (PMD) layer separates metal layer one from the conductive substrate) and separating adjacent active regions of the substrate (e.g., as part of a shallow trench isolation (STI) structure).
For applications such as the deposition of IMD or PMD layers in an integrated circuit or the formation of STI structures, one important physical property of the insulation film is its ability to completely fill gaps between adjacent structures without leaving voids within the gap. This property is referred to as the film's gapfill capability. Gaps that may require filling include spaces between adjacent conductive lines, spaces formed by an etched trench or the like.
As semiconductor device geometries have decreased in size over the years, the ratio of the height of such gaps to their width, the so-called “aspect ratio,” has dramatically increased. Gaps having a combination of a high aspect ratio and a small width present a challenge for semiconductor manufacturers to fill completely. In short, the challenge usually is to prevent the film from forming in a manner that closes off the gap before it is filled. Failure to fill a gap completely results in the formation of a void in the deposited layer, which may adversely affect device operation.
FIG. 1 is a simplified cross-sectional view of a partially completed integrated circuit 10 that can help illustrate the gapfill issue. Partially formed integrated circuit 10 is formed over a silicon substrate 12 that includes a plurality of shallow trench isolation structures 14. As shown in FIG. 1, integrated circuit 10 has a relatively densely packed area 16 where densely packed active devices (e.g., transistors are formed) and a relatively isolated area 18 (also referred to as an “open area”) where an active device may be separated from another active device by a distance that is an order of magnitude more than the spacing between devices in the densely packed area.
A typical shallow trench isolation structure is created by first forming a thin pad oxide layer 20 over the surface of silicon substrate 12 and then form a silicon nitride layer 22 over pad oxide layer 20. The nitride and oxide layers are then patterned using standard photolithography techniques and trenches 24 are etched through the nitride/oxide stack into silicon substrate 12. Trenches 24 are then filled with an insulating material such as silicon dioxide using a deposition process that has good gapfill properties. Prior to the gapfill process, however, an initial lining layer 26, such as an in situ steam generation (ISSG) oxide or other thermal oxide layer or a silicon nitride layer, is usually formed.
In some applications trench 24 has an aspect ratio of between about 6:1 to 8:1 and the formation of a highly conformal film such as oxide liner 26 in trench 24 may increase the aspect ratio even further to, for example 10:1 or higher. Thus, the filling of trenches 24 is typically one of the most challenging gapfill applications in the formation of the integrated circuit.
One known method of depositing thin films, including thin film insulation layers, is referred to as atomic layer deposition or “ALD”. ALD techniques have been known since the 1970's and have been investigated as a way of depositing a variety of materials including silicon oxide. Historically, an ALD process includes repetitively exposing a substrate to alternating flows of different gases, such as source and reactant gases, where a monolayer of the first gas is adsorbed on the surface of the substrate and the second gas reacts with the monolayer to form the desired material. In some instances the chamber is evacuated or purged between flows of the source and reactant gases to remove any excess gas and prevent gas phase reactions from occurring. For example, an ALD process used to form a layer of silicon oxide, includes exposing a substrate to a first silicon-containing gas so that an atomic layer of the silicon-containing gas is adsorbed on the substrate surface, evacuating the chamber to remove any excess silicon-containing gas and then exposing the substrate to an oxidizing agent that oxidizes the layer of silicon-containing material to form a solid thin film layer of silicon oxide. Each cycle of exposing the substrate to a silicon-containing gas followed by an oxidizing agent is then repeated multiple times until a desired film thickness is obtained.
As can be appreciated, such ALD techniques typically result in a very controlled, slow growth of material. Thus, within a semiconductor manufacturing context, ALD techniques have been typically used to form very thin films, e.g., gate oxides, where precise control over film thickness is more important than a high film deposition rate pulse gases, generally slow deposition, not used commercially for oxide gapfill applications.
Despite the efforts of these researches, however, to the best of the inventor's knowledge, no one has developed an ALD silicon oxide process to deposit relatively thick oxide layers such as those required in shallow trench isolation and other gapfill applications, suitable for commercial use.